I'm about to hit 65 and get access to this job ability. I can't find anyone that can tell me that the 50% loss in duration is made up in the additional damage caused. I've already decided to go 5/5 on modus veritas duration and helix attack/accuracy when I hit merit levels. Is there somewhere where I can see the numbers for helix spells cast with and without modus veritas?

I don't think there is as it is random to a point. Your merits are right. I would say go to the dunes and try it out and you will see how much faster mobs drop. Or go to a campain fight if you were 90 and see a mob with and without it.

Just make sure to weather yourself first, and whem MV, use our lvl 40 spell.

Kanican did a whole lot of research on this (specifically around a strategy to kill Absolute Virtue, but the math is still valid), check out the Live Journal entry. There's plenty of good stuff there on Scholars in general at it's a good read for some tips on the job.